JP7261064B2 - IMAGE FORMING APPARATUS, CONTROL METHOD THEREOF, AND PROGRAM - Google Patents

Description

The present invention relates to an image forming apparatus, its control method, and a program.

2. Description of the Related Art An image forming apparatus is known that holds color conversion parameters (profiles) for converting print data (image data) input in RGB format into data in CMYK format. In an image forming apparatus, image processing such as edge enhancement (sharpness) processing is performed on image data in the CMYK format. Japanese Unexamined Patent Application Publication No. 2002-200002 describes a technique for performing edge enhancement processing on CMYK format image data used for image formation in an image forming apparatus. In such an image forming apparatus, the degree of edge enhancement is set so as to be optimal for CMYK format image data output using prestored color conversion parameters.

JP 2015-2443 A

In the image forming apparatus as described above, print data may be input in CMYK format instead of RGB format. For example, print data that has been converted from the RGB format to the CMYK format by some application may be input to the image forming apparatus. In this case, the image forming apparatus cannot specify what kind of color conversion parameter was used to convert from the RGB format to the CMYK format.

If print data input in CMYK format is subjected to the same edge enhancement processing as in the case of input in RGB format, it may not be possible to perform optimum edge enhancement processing. For example, if the print data contains a lot of black components (K components) that are susceptible to contrast-related processing, there is a problem that sharpness is applied too much.

The present invention has been made in view of the above problems. SUMMARY OF THE INVENTION It is an object of the present invention to provide a technique for preventing excessive edge enhancement processing on input print data (image data) depending on the color space of the print data.

An image forming apparatus according to an aspect of the present invention includes a receiving unit that receives input of image data to be printed, and a second color space when the color space of the input image data is a first color space. conversion means for converting into image data in a color space; and edge enhancement processing for the image data after conversion by the conversion means when the color space of the input image data is the first color space. and processing means for performing the edge enhancement process on the input image data when the color space of the input image data is the second color space, the processing means comprising: the processing means for varying the degree of edge enhancement in the edge enhancement process depending on whether the color space is the first color space or the second color space; When the color space of the input image data is the second color space, the degree of edge enhancement in the edge enhancement process is made higher than when the color space of the input image data is the first color space. Characterized by weakening .

According to the present invention, it is possible to prevent excessive edge enhancement processing on input print data (image data) depending on the color space of the print data.

FIG. 2 is a block diagram showing a hardware configuration example of an image forming apparatus; FIG. 2 is a cross-sectional view showing a hardware configuration example of an image forming apparatus; FIG. 2 is a block diagram showing a configuration example of functional blocks corresponding to image processing by an image processing unit; 6 is a flowchart showing the procedure of edge enhancement processing by an edge enhancement unit; 4A and 4B are diagrams showing examples of results of edge enhancement processing; FIG. FIG. 4 is a diagram showing an example of a group of pixels surrounding a pixel of interest, which is subject to accumulation of whiteness; FIG. 4 is a diagram showing an example of a correction value α _CMY for correcting the edge enhancement amount; 4A and 4B are diagrams showing examples of results of edge enhancement processing; FIG. 6 is a flowchart showing the procedure of edge enhancement processing by an edge enhancement unit; FIG. 5 is a diagram showing an example of a correction value α _K for correcting an edge enhancement amount; FIG. 5 is a diagram showing an example of UI for adjusting the degree of edge enhancement;

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. In addition, the following embodiments do not limit the invention according to the scope of claims. Although multiple features are described in the embodiments, not all of these multiple features are essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same or similar configurations are denoted by the same reference numerals, and redundant description is omitted.

[First embodiment]
<Configuration of Image Forming Apparatus>
FIG. 1 is a block diagram showing a hardware configuration example of an image forming apparatus 10 according to this embodiment. The image forming apparatus 10 displays cyan (C), magenta (M), yellow (Ye) (hereinbelow, in order to distinguish from luminance Y, it is expressed as “Ye” only in the case of a single color) and black (K), It has an image forming function (printing function) that forms an image on a sheet using toner of four colors. The image forming apparatus 10 further has various functions such as a reading function of reading (scanning) an image of a document, a copying function of copying an image of a document, and a transmission function of transmitting image data to an external device. .

The image forming apparatus 10 includes an image reading unit 101 , an image processing unit 102 , a storage unit 103 , a CPU 104 , an image output unit 105 , a user interface (UI) unit 106 and a communication unit 107 . The image forming apparatus 10 can be connected to external devices such as a server device that manages image data and a personal computer (PC) that instructs execution of printing via a wired or wireless network.

The storage unit 103 is composed of devices such as a ROM, a RAM, and a hard disk drive (HDD). The ROM stores various control programs and image processing programs executed by the CPU 104 . The RAM is used as a reference area or work area in which the CPU 104 stores various data and information. A color conversion table used for color conversion processing (color separation processing) and other processing parameters for image processing are also stored in the RAM or HDD. By accumulating image data in the RAM and HDD, the CPU 104 can execute processing for, for example, sorting pages, accumulating image data of the sorted document over a plurality of pages, and printing out a plurality of copies. .

The image reading unit 101 reads an image of a document and generates image data. The image output unit 105 forms (outputs) an image on a sheet. Note that the sheet may also be referred to as recording paper, recording material, recording medium, paper, transfer material, transfer paper, and the like. The UI unit 106 includes a display unit that displays various information and an input unit that receives user operations. The input unit is, for example, a touch panel integrated with the display unit. The UI unit 106 receives, for example, an operation for instructing the type or setting of image processing by the image processing unit 102 from the user.

A communication unit 107 is a communication interface capable of communicating with an external device. For example, the communication unit 107 receives print data (image data) from an external device and stores the received print data in the storage unit 103 . The communication unit 107 also transmits print data (image data) stored in the storage unit 103 to an external device.

Print data including image data is input to the image processing unit 102 from the image reading unit 101, the communication unit 107, or the like. The print data is described in a programming language (for example, LIPS or PostScript (registered trademark)) for instructing drawing called a page description language (PDL). Note that LIPS print data is expressed in the RGB color space, and PostScript print data is expressed in the CMYK color space. The image processing unit 102 has an interpreter that interprets print data for each type of PDL.

The image processing unit 102 converts the input print data into intermediate data, and stores (buffers) the intermediate data in a buffer provided in the storage unit 103 . At this time, the image processing unit 102 converts print data described in different types of PDL into common intermediate data. The above buffer is provided in a storage area of RAM or HDD. The image processing unit 102 further generates image data in a color space corresponding to the print data based on the buffered intermediate data, and stores the generated image data in the buffer of the storage unit 103 . At that time, the image processing unit 102 performs color conversion processing, edge enhancement processing, and halftone processing on the image data.

FIG. 2 is a cross-sectional view showing a hardware configuration example of the image forming apparatus 10. As shown in FIG. Here, the configuration and operation of the image forming apparatus 10 will be described by taking as an example copy processing in which the image reading unit 101 reads an image of a document and the image output unit 105 prints the image on a sheet.

In the image reading unit 101 , a document 204 to be read placed between a document platen glass 203 and a document pressure plate 202 is irradiated with light from a lamp 205 . Reflected light from the document 204 is guided to mirrors 206 and 207 and imaged on a 3-line sensor 210 by a lens 208 . Note that the lens 208 is provided with an infrared cut filter 231 . A mirror unit including a mirror 206 and a lamp 205 is driven by a motor (not shown) to move at a speed V in the direction of the arrow. A mirror unit including the mirror 207 is driven by a motor to move in the direction of the arrow at a speed of V/2. In this manner, the entire surface of the document 204 is scanned by moving these mirror units in the direction (sub-scanning direction) perpendicular to the electrical scanning direction (main scanning direction) of the 3-line sensor 210 .

The 3-line sensor 210 is composed of 3-line CCDs 210-1, 210-2, and 210-3. The 3-line sensor 210 converts the incident light into electrical signals, generates image data in the RGB color space consisting of red (R), green (G), and blue (B) color components, and sends the data to the image processing unit 102 . Send. A standard white plate 211 is used to correct image data generated by each CCD of the 3-line sensor 210 .

The image processing unit 102 performs color conversion processing on the image data in the RGB color space input from the 3-line sensor 210 . Specifically, the image processing unit 102 extracts image data of each color component of cyan (C), magenta (M), yellow (Ye), and black (K) from the input image data (image data in the CMYK color space). ). Further, the image processing unit 102 performs halftone processing on the generated image data, and transmits the image data on which the halftone processing has been performed to the image output unit 105 .

In the image output unit 105 , the image data in the CMYK color space received from the image processing unit 102 is input to the laser driver 212 . The laser driver 212 drives the semiconductor laser element 213 according to each color component data of the input image data. A laser beam output from the semiconductor laser element 213 is irradiated onto the photosensitive drum 217 via the polygon mirror 214 , the fθ lens 215 and the mirror 216 . As a result, an electrostatic latent image corresponding to the image data input to the photosensitive drum 217 is formed on the photosensitive drum 217 .

Developing units 219 to 222 develop the electrostatic latent image formed on the photosensitive drum 217 using C, M, Ye, and K toners, respectively. Thereby, a toner image is formed on the photosensitive drum 217 . A sheet supplied from the paper feed cassette 225 is transported along the transport path and wound around the transfer drum 223 . The toner images of C, M, Ye, and K on the photosensitive drum 217 are superimposed and transferred onto the sheet in order. The sheet on which the multicolor image is thus formed is subjected to fixing processing by the fixing unit 226, and then discharged to a paper discharge tray outside the apparatus.

<Functional Configuration of Image Processing Unit>
FIG. 3 is a block diagram showing a functional block configuration example corresponding to image processing by the image processing unit 102 according to this embodiment. As shown in FIG. 3, the image processing unit 102 receives image data in the RGB color space (first color space) and image data in the CMYK color space (second color space) as input of image data to be printed in the image forming apparatus 10. ) and accepts any input with image data. As described above, when image data is input from the 3-line sensor 210 , image data in the RGB color space is input to the image processing unit 102 . Also, when print data described in the RGB color space is received from an external device, image data in the RGB color space is input to the image processing unit 102 .

When image data in the RGB color space is input, the image processing unit 102 executes color conversion processing before executing edge enhancement processing. A color conversion unit 301 converts image data in the RGB color space into image data in the CMYK color space, which consists of four color components corresponding to toner colors. This conversion is performed using a color conversion table (color conversion parameters) stored in the storage unit 103 (HDD). In this manner, when the color space of input image data to the image processing unit 102 (image forming apparatus 10) is the RGB color space (first color space), the color conversion unit 301 converts the image data into the CMYK color space. (Second color space) image data.

An edge enhancement unit 302 performs edge enhancement processing, which will be described later, on image data in the CMYK color space. As shown in FIG. 3, when the color space of the input image data is the RGB color space (first color space), the edge enhancement unit 302 converts the image in the CMYK color space after conversion by the color conversion unit 301. Edge enhancement processing is performed on the data. On the other hand, when the color space of the input image data is the CMYK color space (second color space), the edge enhancement unit 302 performs edge enhancement processing on the input image data.

In this embodiment, the edge enhancement unit 302 determines the degree of edge enhancement in the edge enhancement process depending on whether the color space of the input image data is the RGB color space or the color space of the input image data is the CMYK color space. be different. This makes it possible to prevent excessive edge enhancement processing on input print data (image data) depending on the color space of the print data.

As will be described later, edge enhancement processing by the edge enhancement unit 302 includes filtering processing for luminance signals obtained from image data to be processed. In this embodiment, as will be described later with reference to FIG. 4, an example will be described in which the degree of edge enhancement is adjusted by adjusting the filter gain in the filtering process according to the color space of the input image data. When the color space of the input image data is the CMYK color space (second color space), the edge enhancement unit 302 performs filtering more than when the color space of the input image data is the RGB color space (first color space). Decrease the gain. As a result, when the color space of the input image data is the CMYK color space (the second color space), the edge enhancement unit 302 can perform the edge enhancement more effectively than when the color space of the input image data is the RGB color space (the first color space). also weakens the degree of edge enhancement in edge enhancement processing.

Image data that has undergone edge enhancement processing is output from the edge enhancement unit 302 to the halftone processing unit 303 . A halftone processing unit 303 performs halftone processing using a dither method, for example. Image data after halftone processing is transmitted to the image output unit 105 or stored in the storage unit 103 .

<Edge enhancement processing>
FIG. 4 is a flow chart showing the procedure of edge enhancement processing executed by the edge enhancement unit 302. As shown in FIG. The processing of each step shown in FIG. 4 is executed by the edge enhancement unit 302 (image processing unit 102) according to instructions from the CPU 104. FIG.

Here, as an example, the filter gain in the filtering process for the luminance signal (L signal) obtained from the color component signal (K signal) corresponding to black among the color component signals included in the image data to be processed is the input image. Adjust according to the color space of the data. This makes it possible to prevent excessive edge enhancement (sharpness) processing from being performed on the K signal, which is particularly susceptible to contrast-related processing.

First, in S401, the edge enhancement unit 302 divides the input image data in the CMYK color space into CMY signals, which are color component signals corresponding to colors other than black, and K signals, which are color component signals corresponding to black. To separate. Furthermore, the edge enhancement section 302 converts the CMY signals into RGB signals and converts the K signals into L signals. For example, when the signal values (C, M, Ye, K) of each color of the input image data are composed of 8 bits (they are signal values of 256 gradations), the RGB signal and the L signal ( luminance) into signal values.
R = 255 - C
G=255-M
B = 255 - Ye
L = 255 - K (1)
Note that the numerical value "255" in equation (1) is a value determined as (2 ^d -1) according to the number of bits d.

The RGB signals of each pixel obtained in S401 are processed in S402 to S408. The L signal of each pixel obtained in S401 is processed in S410 to S415. The processes of S402 to S408 and the processes of S410 to S415 are respectively executed by sequentially setting each pixel forming the image data as the pixel of interest. The processing of S402 to S408 and the processing of S410 to S415 may be executed in parallel or sequentially.

(Edge enhancement processing for RGB signals)
Signal values (R', G', B') after edge enhancement are obtained from the signal values (R, G, B) of the RGB signals by the processing of S402 to S408.

First, in S402, the edge enhancement unit 302 performs color conversion on the RGB signal into a luminance/color difference color space. In this embodiment, for the target pixel, the signal values (R, G, B) of the RGB signals are converted into the signal values (Y, Cb, Cr) of the YCbCr color space as shown in the following equations.
Y = 0.2990*R+0.5870*G+0.1140*B
Cb=-0.1687*R-0.3313*G+0.5000*B
Cr = 0.5000*R-0.4187*G-0.0813*B (2)
When the signal value of each color of the RGB signal is composed of 8 bits, the Y signal (luminance) has a value of 0 to 255, the Cb signal (color difference) has a value of -128 to 127, and the Cr signal (color difference) has - It has a value of 128-127. This conversion converts the RGB signals into Y, Cb and Cr signals.

Next, in S403, the edge enhancement unit 302 extracts signals of a plurality of pixels including the pixel of interest and its surrounding pixels within a window (filter window) of a predetermined size from the luminance signal (Y) and the color difference signal (CbCr). Extract values. In this embodiment, a filter window with a size of 5×5 pixels is used as an example.

Furthermore, in S404, the edge enhancement unit 302 performs edge enhancement by performing filter convolution (filtering) with a predetermined filter coefficient on the signal values within the filter window of the Y signal. Through this filtering process, a Y' signal, which is a luminance signal after edge enhancement, is obtained from the Y signal. In the present embodiment, the filter processing in S404 is the first filter processing for the luminance signal (Y signal) obtained from the color component signals corresponding to colors other than black among the color component signals included in the image data to be processed. An example. The edge enhancement unit 302 makes the same filter gain in the filtering process in S404 regardless of the color space of the input image data.

このフィルタ係数行列のサイズは、Ｓ４０３で使用されるフィルタウィンドウサイズ以下とされる必要がある。本実施形態では、Ｓ４０４の処理は、輝度信号（Ｙ）に対してのみ行われ、色差信号（ＣｂＣｒ）に対しては行われない。これにより、画像の明るさのみが強調され、画像のエッジ部分における色変化を抑えることが可能になる。 As an example of the filtering process in S404, there is a process of calculating the secondary differential component using a Laplacian filter and subtracting the obtained value from the signal value of the original image. When implementing this process by one convolution, for example, filter coefficients represented by a matrix as shown below are used.

The size of this filter coefficient matrix should be less than or equal to the filter window size used in S403. In this embodiment, the process of S404 is performed only on the luminance signal (Y) and not on the color difference signal (CbCr). As a result, only the brightness of the image is emphasized, making it possible to suppress color change in the edge portion of the image.

Here, FIG. 5 is a diagram showing an example of the processing result of S404. The horizontal axis of the graph shown in FIG. 5 indicates the position of the pixel, and the vertical axis indicates the brightness of each pixel. As shown in FIG. 5, a change occurs in the Y' signal after edge enhancement near the change point of the Y signal. Specifically, the Y' signal after edge enhancement is contrasted such that darker portions of the image are darker and brighter portions are brighter.

When the process of S404 is completed, next in S405, the edge enhancement unit 302 obtains the cumulative value of the whiteness of the pixels surrounding the pixel of interest within the filter window. This cumulative value is used to correct the edge enhancement amount in S406 and S407.

In this embodiment, a pixel with low saturation and high luminance is defined as a pixel with high whiteness, and the whiteness is set so that such a pixel has a high whiteness value. Specifically, the saturation S is calculated using a color difference signal (CbCr). As described above, the color difference signal (CbCr) indicates a color component, and the distance from the signal value (Cb, Cr)=(0, 0) expresses the vividness (saturation) of the color. Therefore, the saturation S is obtained by the following equation.
S=(Cb ² +Cr ² ) ^1/2 (3)

Furthermore, the whiteness W of a certain pixel is, for example, the saturation S obtained by the above equation, the L signal (luminance) obtained from the K signal in S401, and the Y signal (luminance) obtained from the RGB signals in S402. is obtained from the following equation using
W=Y ² +L ² +(255-S) ² (4)
Here, the larger the value of the Y signal and the L signal, the higher the luminance, and the larger the value of the saturation S, the higher the saturation. On the other hand, (255-S) indicates that the larger the value, the lower the saturation. Therefore, the whiteness W of a certain pixel has a larger value as the brightness of the pixel is higher, and a larger value as the saturation is lower. Considering that the bright achromatic color is white, it will be appreciated that W in equation (4) indicates whiteness. Since the L signal is the complement of the K signal, it can be understood that the whiteness W is determined based on the Y signal (luminance) and the K signal.

In S405, the edge enhancement unit 302 obtains the whiteness W for a predetermined number of pixels (pixel group) around the pixel of interest, and obtains the cumulative value. FIG. 6 shows an example of obtaining the cumulative value of whiteness W for 16 pixels around the target pixel. In the example of FIG. 6, the whiteness W is obtained for each of the 16 pixel groups 602 indicated by the symbol "x" around the pixel of interest 601, and the cumulative value ΣW thereof is obtained. According to this example, for example, in the pixel group 602 arranged in a square, the cumulative value obtained when only the pixels forming one side of the square are white is four times the cumulative value when all the pixels are white. will be obtained. In this way, the integrated value ΣW of the whiteness W is obtained as the degree of whiteness of the pixel group surrounding the pixel of interest.

Next, in S406, the edge enhancement unit 302 generates a correction value α _CMY for correcting the edge enhancement amount according to the cumulative value ΣW obtained in S405. The correction value α _CMY can take values from 0 to 1.0. α _CMY =1.0 does not require correction, and is defined such that the smaller the value of α _CMY , the larger the amount of correction. For example, as shown in FIG. 7, α _CMY is generated so that it approaches 1.0 as the cumulative value ΣW decreases, and decreases (approaches 0) as the cumulative value ΣW increases. Edge enhancement section 302 acquires correction value α _CMY corresponding to cumulative value ΣW using the correspondence shown in FIG. 7 . Note that the correction value α _CMY can be generated by using a lookup table (LUT) that receives the accumulated value ΣW and outputs α _CMY . This LUT can be stored in the storage unit 103 in advance and read out from the storage unit 103 for use.

Furthermore, in S407, the edge enhancement unit 302 performs edge enhancement based on the correction value α _CMY obtained in S406, the Y signal of the original image obtained in S402, and the Y′ signal after edge enhancement obtained in S404. Perform amount correction processing. The Y'' signal, which is the luminance signal after correction, is obtained by weighting the Y signal and the Y' signal after edge enhancement according to α _CMY as shown in the following equation, and synthesizing the weighted signals. obtained by
Y''=(1.0- _αCMY )*Y+ _αCMY *Y' (5)

In this manner, the edge enhancement unit 302 combines the brightness signal (Y signal) before the filtering process in S404 and the brightness signal (Y' signal) obtained by the filtering process to correct the edge enhancement amount. and are respectively weighted and synthesized.

After that, in S408, the edge enhancement unit 302 performs inverse conversion from the YCbCr color space to the RGB color space on the signal after the correction processing in S407. Specifically, as shown in the following equation, the signal values (R′, G′, B′) after edge enhancement for the pixel of interest are obtained by performing the inverse matrix calculation of the color conversion in S402.
R′=Y″+1.4020*Cr
G′=Y″−0.3441*Cb−0.7141*Cr
B'=Y''+1.7720*Cb (6)

(Edge enhancement processing for L signal)
On the other hand, through the processing of S410 to S415, the signal value of the L'' signal, which is the luminance signal after edge enhancement, is obtained from the signal value of the L signal.

First, in S410, the edge enhancement unit 302 extracts signal values of a plurality of pixels including the pixel of interest and surrounding pixels within a filter window of a predetermined size from the L signal (luminance), as in S403.

Next, in step S411, the edge enhancement unit 302 determines whether the color space (input color space) of the image data (print data) input to the image processing unit 102 is the RGB color space or the CMYK color space. . The edge enhancement unit 302 advances the process to S412 if the input color space is the RGB color space, and advances the process to S413 if it is the CMYK color space.

If the color space of the input image data is the RGB color space, in S412 the edge enhancement unit 302 performs edge enhancement on the L signal in the same manner as in S404. Specifically, the edge enhancement unit 302 performs filter convolution (filtering) with a predetermined filter coefficient on the signal value within the filter window of the L signal, thereby converting the L signal into the edge-enhanced luminance signal. Find some L' signal.

On the other hand, if the color space of the input image data is the CMYK color space, in S413 the edge enhancement unit 302 uses a filter coefficient different from that in S412 to perform edge enhancement on the L signal, thereby increasing the edge Find the L' signal after enhancement. Specifically, the edge enhancement unit 302 uniformly multiplies each filter coefficient used in S412 by a gain adjustment value G for adjusting the filter gain in the filtering process. This changes the degree of edge enhancement by filtering using the filter coefficient. In this embodiment, as an example, the gain adjustment value G is set to 0.5. As a result, when the color space of the image data is the CMYK color space, the degree of edge enhancement for the K signal is suppressed compared to when the color space of the image data is the RGB color space.

なお、このフィルタ係数行列を使用した場合、Ｌ'信号の値が全体的に低くなり、暗い画像が得られることになるため、フィルタ係数行列の中央に位置する係数（１．０４）を、当該行列内の係数の総和が１．０となるように調整（オフセット）する。本例では、中央に位置する係数に０．５を加算して得られた、以下のフィルタ係数行列が、Ｓ４１３の処理に使用される。なお、ゲイン調整値Ｇが適用された上記のフィルタ係数行列が予め用意され、記憶部１０３に格納されていてもよい。また、ゲイン調整値Ｇは、１．０未満の値であれば、０．５以外の値であってもよい。 For example, when the gain adjustment value G=0.5, the following filter coefficient matrix is obtained by multiplying the gain adjustment value G by the filter coefficients used in S404 and S412.

Note that when this filter coefficient matrix is used, the value of the L' signal is generally low, resulting in a dark image. Adjust (offset) so that the sum of the coefficients in the matrix is 1.0. In this example, the following filter coefficient matrix obtained by adding 0.5 to the centrally located coefficient is used for the processing of S413. Note that the filter coefficient matrix to which the gain adjustment value G is applied may be prepared in advance and stored in the storage section 103 . Also, the gain adjustment value G may be a value other than 0.5 as long as it is less than 1.0.

In the present embodiment, the filtering process of S412 and S413 is an example of second filtering process for the luminance signal (L signal) obtained from the color component signal corresponding to black. In order to adjust the degree of edge enhancement according to the color space of the input image data, the edge enhancement unit 302 adjusts the filter gain in the filtering process according to the color space of the input image data as described above.

Here, FIG. 8 is a diagram showing an example of the processing results of S412 and S413. The horizontal axis of the graph shown in FIG. 8 indicates the pixel position, and the vertical axis indicates the luminance of each pixel. As shown in FIG. 8, near the change point of the L signal (luminance), there is no change in the L' signal after edge enhancement in both the case where the input color space is the RGB color space and the case where the input color space is the CMYK color space. is occurring. Specifically, the L' signal after edge enhancement is contrasted such that the dark portions of the image are darker and the bright portions are brighter. The L' signal when the input color space is the CMYK color space has a smaller change than the L' signal when the input color space is the RGB color space, which indicates the degree of edge enhancement. is suppressed.

According to such processing, even if an external device performs unexpected color conversion on print data (image data) expressed in the CMYK color space, the image processing unit 102 can convert the K signal into Excessive edge enhancement can be prevented.

Next, in S414, the edge enhancement unit 302 generates a correction value α _K for correcting the edge enhancement amount in accordance with the whiteness cumulative value ΣW obtained in S405, as in S406. Furthermore, in S415, the edge enhancement unit 302 extracts the correction value α _K obtained in S414, the L signal (luminance) of the original image obtained in S401, and the L signal after edge enhancement obtained in S412, as in S407. 'Based on the signal, the amount of edge enhancement is corrected. The corrected L'' signal is obtained by weighting the L signal and the edge-enhanced L' signal according to α _K and synthesizing the weighted signals, as shown in the following equation. .
L″=(1.0−α _K )*L+α _K *L′ (7)

In this manner, the edge enhancement unit 302 combines the luminance signal (L signal) before the filtering in S412 or S413 and the luminance signal (L' signal) are weighted and combined.

When the processes of S408 and S415 are completed, in S409, the edge enhancement unit 302 converts the signal values (R', G', B', L'') obtained by the above-described processes into the following equations of S401. By the inverse transformation of the transformation, the signal values are transformed into the CMYK signals after the edge enhancement processing.
C'=255-R'
M'=255-G'
Ye'=255-B'
K'=255-L'' (8)

By performing the above-described processing on all pixels of the input image data, edge-enhanced image data is generated.

As described above, in the image forming apparatus 10 of the present embodiment, the image processing unit 102 receives input of image data to be printed. When the color space of input image data is the RGB color space (first color space), the color conversion unit 301 converts the image data into image data in the CMYK color space (second color space). The edge enhancement unit 302 performs edge enhancement processing on the image data converted by the color conversion unit 301 when the color space of the input image data is the RGB color space. On the other hand, when the color space of the input image data is the CMYK color space, the edge enhancement unit 302 performs edge enhancement processing on the input image data.

In the edge enhancement processing described above, the edge enhancement unit 302 differentiates the degree of edge enhancement in the edge enhancement processing depending on whether the color space of the input image data is the RGB color space or the CMYK color space. More specifically, when the color space of the input image data is the CMYK color space, the edge enhancement unit 302 performs edge enhancement in edge enhancement processing more than when the color space of the input image data is the RGB color space. weaken the intensity. This makes it possible to prevent excessive edge enhancement processing on the input print data (image data) depending on the color space of the print data.

In the edge enhancement process described above, the filter coefficient applied to the L signal (luminance) generated from the K signal is set to the gain adjustment value according to the color space of the input print data (input color space). is adjusted by That is, it adjusts the filter gain in the filtering process. This prevents excessive edge enhancement (sharpness) processing, especially for the K signal, which has the lowest toner brightness and is susceptible to contrast-related processing. In addition, by making the filter coefficients applied to the Y signal (luminance) generated from the CMY signals common (same) regardless of the color space of the input print data, The effect of edge enhancement can be maintained for print data.

It should be noted that this embodiment can be modified in various ways. For example, in the present embodiment, the YCbCr color space of the luminance/color difference system is used as the color space for edge enhancement processing, but another color space of the luminance/chrominance system (for example, the L*a*b* color space) is used. may be used. Also, for the edge enhancement processing, a technique other than the technique described above (for example, unsharp mask) may be used.

Also, although the whiteness W is calculated using the saturation S and the Y signal (luminance) in S405, only the Y signal may be used. This is because the RGB signal components are sufficiently reflected in the Y signal, and if the value of the Y signal of a pixel is sufficiently large, the pixel can be regarded as white. In this case, for example, instead of ΣW in S405, the accumulated value ΣY of the signal values of the Y signals of the pixel group surrounding the target pixel may be obtained, and the correction value α _CMY corresponding to the accumulated value ΣY may be obtained. Further, a predetermined constant value independent of ΣW may be used as the correction value α _K applied to the L signal (luminance) in S415.

Further, in this embodiment, the filter coefficients applied to the Y signal (luminance) generated from the CMY signals are common regardless of the color space of the input print data, but the present invention is not limited to this. For example, when the color space of input print data is the CMYK color space, the degree of edge enhancement suppressed for the K signal may be compensated for by edge enhancement for the CMY signal. In that case, for example, by applying the reciprocal of the gain adjustment value G to the filter coefficient applied to the Y signal generated from the CMY signals, the color space of the print data is more accurate than when the color space of the print data is the RGB color space. The degree of edge enhancement may be strengthened.

[Second embodiment]
In the first embodiment, when the color space (input color space) of the image data (print data) input to the image processing unit 102 is the CMYK color space, edge adjustment is performed by applying the gain adjustment value G to the filter coefficients. The configuration for correcting the enhancement amount has been described. In this embodiment, a configuration will be described in which the edge enhancement amount is corrected by using different correction values α _K depending on whether the input color space is the RGB color space or the CMYK color space. Note that the description of the parts common to the first embodiment will be omitted.

FIG. 9 is a flowchart showing the procedure of edge enhancement processing according to this embodiment. The processing of each step shown in FIG. 9 is executed by the edge enhancement unit 302 (image processing unit 102) according to instructions from the CPU 104. FIG.

In this embodiment, the edge enhancement unit 302 advances the process to S412 after performing the process of S410. That is, in S412, the edge enhancement unit 302 performs the same filter processing as in S404 on the L signal (luminance) without changing the filter coefficient according to the input color space, and proceeds to S405.

Further, after the processing of S412 is completed, the edge enhancement unit 302 advances the processing to S901 when the whiteness cumulative value is calculated for the pixel group surrounding the pixel of interest in S405. In S901, the edge enhancement unit 302 determines whether the color space (input color space) of the image data (print data) input to the image processing unit 102 is the RGB color space or the CMYK color space. The edge enhancement unit 302 advances the process to S414 if the input color space is the RGB color space, and advances the process to S902 if it is the CMYK color space.

In S414, as in the first embodiment, the edge enhancement unit 302 generates a correction value α _K for correcting the edge enhancement amount according to the whiteness cumulative value ΣW obtained in S405, and the process proceeds to S415. proceed. On the other hand, in S902, the edge enhancement unit 302 generates a correction value different from that in S414 as the correction value α _K for correcting the edge enhancement amount. Specifically, the edge enhancement unit 302 generates the correction value α _K such that the maximum value of α _K is lower than 1.0, as shown in FIG. 10 . In the example of FIG. 10, the maximum value of the correction value α _K is 0.5, which is half the maximum value of 1.0 in the example of FIG. Edge enhancement section 302 acquires correction value α _K corresponding to cumulative value ΣW using the correspondence shown in FIG. 10 .

Thereafter, in S415, the edge enhancement unit 302 uses the correction value α _K generated in S414 or S902 to perform edge enhancement amount correction processing for the pixel of interest. As a result, as shown in FIG. 8, when the input color space is the CMYK color space, the degree of edge enhancement for the L signal generated from the K signal is higher than when the input color space is the RGB color space. is suppressed. That is, the same results as in the first embodiment can be obtained.

As described above, in the present embodiment, the edge enhancement unit 302 (image processing unit 102) is applied to the L signal generated from the K signal according to the color space of the input print data (input color space). Adjust the correction value α _K to be applied. That is, the edge enhancement unit 302 adjusts the degree of edge enhancement by adjusting the weighting coefficient (correction value α _K ) used in the correction process for correcting the edge enhancement amount according to the color space of the input image data. adjust. As a result, as in the first embodiment, it is possible to prevent excessive edge enhancement processing for input print data (image data) depending on the color space of the input print data. Also, it is possible to prevent the K signal from being subjected to excessive edge enhancement processing.

Note _that in the present embodiment, an example of adjusting the correction value α _K for correcting the edge enhancement amount has been described. may be performed.

Further, in the present embodiment, the correction value _CMY applied to the Y signal (luminance) generated from the CMY signal is common regardless of the color space of the input print data, but is not limited to this. . For example, when the color space of input print data is the CMYK color space, the degree of edge enhancement suppressed for the K signal may be compensated for by edge enhancement for the CMY signal. In that case, for example, when the input color space is the CMYK color space, the correction value α _CMY applied to the Y signal generated from the CMY signals is relatively By increasing it, the degree of edge enhancement may be strengthened.

[Third Embodiment]
In the first embodiment, a configuration using different filter coefficients for each color space (input color space) of image data (print data) input to the image processing unit 102 has been described. In this embodiment, a configuration will be described in which filter coefficients can be changed according to setting values specified by a user via a user interface (UI). Note that the description of the parts common to the first embodiment will be omitted.

FIG. 11 is a diagram showing an example of an operation screen (UI) displayed on the UI unit 106. As shown in FIG. As shown in FIG. 11, the UI 1100 is configured such that the user can set the degree of edge enhancement by operating a slide bar 1101 . The CPU 104 stores the setting value indicating the setting in the UI 1100 in the storage unit 103, thereby setting the degree of edge enhancement according to the user's operation. In the UI 1100, as an example, the degree of edge strength is set in four levels ("no", "weak", " It can be set to either "medium" or "strong").

The edge enhancement unit 302 (image processing unit 102) adjusts filter coefficients used in S404, S412, and S413 according to setting values set via the UI 1100. FIG. Specifically, the edge enhancement unit 302 adjusts the filter coefficients by multiplying each filter coefficient in the filter coefficient matrix by the set value. In order to prevent the edge-adjusted image from becoming a dark image due to this adjustment, as described in the first embodiment, the coefficient located in the center of the filter coefficient matrix is changed to It may be adjusted (offset) so that the sum is 1.0.

As described above, in this embodiment, the edge enhancement unit 302 adjusts the degree of edge enhancement in accordance with user settings. As a result, in addition to the same effects as in the first embodiment, it is possible to change the degree of edge enhancement according to the user's preference.

Note that in this embodiment, the gain adjustment value G applied when the input color space is the CMYK color space does not depend on the setting in the UI 1100, but is not limited to this. For example, the gain adjustment value G may be adjusted so that the gain adjustment value G approaches 1.0 as the degree of edge enhancement indicated by the setting value set via the UI 1100 increases. Further, when the maximum degree of edge enhancement is specified as the settable edge enhancement degree, the gain adjustment value G is set to 1.0 in order to obtain the maximum edge enhancement effect. You may make it not suppress a degree.

[Other embodiments]
The present invention supplies a program that implements one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and one or more processors in the computer of the system or device reads and executes the program. It can also be realized by processing to It can also be implemented by a circuit (for example, ASIC) that implements one or more functions.

The invention is not limited to the embodiments described above, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the claims are appended to make public the scope of the invention.

10: Image forming apparatus 101: Image reading unit 102: Image processing unit 103: Storage unit 104: CPU 105: Image output unit 106: UI unit 107: Communication unit 301: Color conversion unit 302 : edge enhancement unit, 303: halftone processing unit

Claims (19)

receiving means for receiving input of image data to be printed;
conversion means for converting the input image data into image data in a second color space when the color space of the input image data is in the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion by the conversion means, and the color space of the input image data is the first color space. When the color space of the input image data is the first color space, and when the color space of the input image data is the first color space. the processing means for differentiating the degree of edge enhancement in the edge enhancement process depending on whether it is the second color space,
When the color space of the input image data is the second color space, the processing means performs the edge enhancement processing more than when the color space of the input image data is the first color space. and weakening the degree of edge enhancement in the image forming apparatus. receiving means for receiving input of image data to be printed;
conversion means for converting the input image data into image data in a second color space when the color space of the input image data is in the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion by the conversion means, and the color space of the input image data is the first color space. When the color space of the input image data is the first color space, and when the color space of the input image data is the first color space. the processing means for differentiating the degree of edge enhancement in the edge enhancement process depending on whether it is the second color space,
The edge enhancement processing includes filtering a luminance signal obtained from the image data to be processed,
The image forming apparatus, wherein the processing unit adjusts the degree of edge enhancement by adjusting a filter gain in the filtering process according to the color space of the input image data. The processing means converts the filter gain in the filtering process for the luminance signal obtained from the color component signal corresponding to black among the color component signals included in the image data to be processed to the color of the input image data. 3. The image forming apparatus according to claim 2 , wherein the adjustment is made according to the space. When the color space of the input image data is the second color space, the processing means increases the filter gain more than when the color space of the input image data is the first color space. 4. The image forming apparatus according to claim 2 , wherein the image forming apparatus is made smaller. The edge enhancement process includes a first filtering process on a luminance signal obtained from color component signals corresponding to colors other than black among the color component signals included in the image data to be processed; a second filtering of the resulting luminance signal;
The processing means sets the filter gain in the first filtering process to be the same regardless of the color space of the input image data, and sets the filter gain in the second filtering process to the color space of the input image data. 5. The image forming apparatus according to claim 2 , wherein the adjustment is made accordingly. receiving means for receiving input of image data to be printed;
conversion means for converting the input image data into image data in a second color space when the color space of the input image data is in the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion by the conversion means, and the color space of the input image data is the first color space. When the color space of the input image data is the first color space, and when the color space of the input image data is the first color space. the processing means for differentiating the degree of edge enhancement in the edge enhancement processing in the second color space and in the case of the second color space;
setting means for setting the degree of edge enhancement according to a user's operation;
The image forming apparatus, wherein the processing means adjusts the degree of edge enhancement according to the setting by the setting means. receiving means for receiving input of image data to be printed;
conversion means for converting the input image data into image data in a second color space when the color space of the input image data is in the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion by the conversion means, and the color space of the input image data is the first color space. When the color space of the input image data is the first color space, and when the color space of the input image data is the first color space. the processing means for differentiating the degree of edge enhancement in the edge enhancement process depending on whether it is the second color space,
the first color space is an RGB color space;
The image forming apparatus, wherein the second color space is a CMYK color space. receiving means for receiving input of image data to be printed;
conversion means for converting the input image data into image data in a second color space when the color space of the input image data is in the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion by the conversion means, and the color space of the input image data is the first color space. When the color space of the input image data is the first color space, and when the color space of the input image data is the first color space. the processing means for differentiating the degree of edge enhancement in the edge enhancement processing in the second color space and in the case of the second color space;
printing means for printing an image on a sheet based on the image data in the second color space on which the edge enhancement processing has been performed by the processing means ;
An image forming apparatus comprising : The edge enhancement processing includes filtering a luminance signal obtained from the image data to be processed, and correcting the amount of edge enhancement by filtering the luminance signal before the filtering and the luminance signal obtained by the filtering. and a correction process that weights and synthesizes each,
9. The method according to claim 8 , wherein the processing means adjusts the degree of edge enhancement by adjusting a weighting coefficient used in the correction process according to the color space of the input image data. image forming device. a receiving step of receiving an input of image data;
a converting step of converting the input image data into image data of a second color space when the color space of the input image data is the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion in the conversion step, and the color space of the input image data is the first color space. When the color space of the input image data is the second color space, the processing step of performing the edge enhancement processing on the input image data, wherein the color space of the input image data is the first color space. and the processing step of differentiating the degree of edge enhancement in the edge enhancement processing in the case of the second color space,
In the processing step, when the color space of the input image data is the second color space, the edge enhancement processing is performed more than when the color space of the input image data is the first color space. weaken the degree of edge enhancement in
A control method for an image forming apparatus, characterized by: a receiving step of receiving an input of image data;
a converting step of converting the input image data into image data of a second color space when the color space of the input image data is the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion in the conversion step, and the color space of the input image data is the first color space. When the color space of the input image data is the second color space, the processing step of performing the edge enhancement processing on the input image data, wherein the color space of the input image data is the first color space. and the processing step of differentiating the degree of edge enhancement in the edge enhancement processing in the case of the second color space,
The edge enhancement processing includes filtering a luminance signal obtained from the image data to be processed,
In the processing step, the degree of edge enhancement is adjusted by adjusting a filter gain in the filtering process according to the color space of the input image data.
A control method for an image forming apparatus, characterized by: In the processing step, the filter gain in the filtering process for a luminance signal obtained from a color component signal corresponding to black among the color component signals included in the image data to be processed is set to the color of the input image data. Adjust according to space
12. The method of controlling an image forming apparatus according to claim 11, wherein: In the processing step, when the color space of the input image data is the second color space, the filter gain is set higher than when the color space of the input image data is the first color space. make smaller
13. The method of controlling an image forming apparatus according to claim 11, wherein: The edge enhancement process includes a first filtering process on a luminance signal obtained from color component signals corresponding to colors other than black among the color component signals included in the image data to be processed; a second filtering of the resulting luminance signal;
In the processing step, the filter gain in the first filtering process is set to be the same regardless of the color space of the input image data, and the filter gain in the second filtering process is set to the color space of the input image data. adjust accordingly
14. The method of controlling an image forming apparatus according to claim 11, wherein: a receiving step of receiving an input of image data;
a converting step of converting the input image data into image data of a second color space when the color space of the input image data is the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion in the conversion step, and the color space of the input image data is the first color space. When the color space of the input image data is the second color space, the processing step of performing the edge enhancement processing on the input image data, wherein the color space of the input image data is the first color space. the processing step of differentiating the degree of edge enhancement in the edge enhancement processing in the case of the second color space;
a setting step of setting the degree of edge enhancement according to a user's operation;
In the processing step, the degree of edge enhancement is adjusted according to the setting in the setting step.
A control method for an image forming apparatus, characterized by: a receiving step of receiving an input of image data;
a converting step of converting the input image data into image data of a second color space when the color space of the input image data is the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion in the conversion step, and the color space of the input image data is the first color space. When the color space of the input image data is the second color space, the processing step of performing the edge enhancement processing on the input image data, wherein the color space of the input image data is the first color space. and the processing step of differentiating the degree of edge enhancement in the edge enhancement processing in the case of the second color space,
the first color space is an RGB color space;
The second color space is the CMYK color space
A control method for an image forming apparatus, characterized by: a receiving step of receiving an input of image data;
a converting step of converting the input image data into image data of a second color space when the color space of the input image data is the first color space;
When the color space of the input image data is the first color space, edge enhancement processing is performed on the image data after conversion in the conversion step, and the color space of the input image data is the first color space. When the color space of the input image data is the second color space, the processing step of performing the edge enhancement processing on the input image data, wherein the color space of the input image data is the first color space. the processing step of differentiating the degree of edge enhancement in the edge enhancement processing in the case of the second color space;
a printing step of printing an image on a sheet based on the image data in the second color space on which the edge enhancement processing in the processing step has been performed;
A control method for an image forming apparatus, comprising: The edge enhancement processing includes filtering a luminance signal obtained from the image data to be processed, and correcting the amount of edge enhancement by filtering the luminance signal before the filtering and the luminance signal obtained by the filtering. and a correction process that weights and synthesizes each,
In the processing step, the degree of edge enhancement is adjusted by adjusting the weighting coefficient used in the correction process according to the color space of the input image data.
18. The method of controlling an image forming apparatus according to claim 17, wherein: A program for causing a computer to execute each step of the image forming apparatus control method according to any one of claims 10 to 18 .

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